Implement billing functionality and enhance role management

- Introduced billing capabilities, allowing users to request payments for subscription periods (3/6/12 months) with admin approval via Telegram.
- Updated role management to include a `BillingEnabled` property, preventing billing for admin roles.
- Enhanced the `CreateRoleCommand` and `UpdateRoleCommand` to accept billing parameters, ensuring proper handling during role creation and updates.
- Added new endpoints for billing management and integrated billing checks into VPN config creation to enforce payment requirements.
- Updated related services, models, and tests to support the new billing features, ensuring comprehensive coverage and functionality.
- Enhanced documentation to reflect the new billing processes and role management changes.

backend/src/PnvPanel.Api/Telegram/PnvBotUpdateHandler.cs

@@ -1,5 +1,6 @@
 using Microsoft.Extensions.Options;
 using PnvPanel.Application.Admin.Activation;
+using PnvPanel.Application.Admin.Billing;
 using PnvPanel.Application.Admin.Support;
 using PnvPanel.Application.Common.Interfaces;
 using PnvPanel.Application.Common.Messaging;
@@ -442,6 +443,55 @@ public sealed class PnvBotUpdateHandler(
 
                 break;
             }
+            case "pay":
+            {
+                if (!await TrySetAdminCurrentUserAsync(services, fromId, cancellationToken))
+                {
+                    await botClient.AnswerCallbackQuery(
+                        callback.Id,
+                        "Недостаточно прав.",
+                        cancellationToken: cancellationToken
+                    );
+                    return;
+                }
+
+                var result =
+                    parts[1] == "approve"
+                        ? await sender.Send(
+                            new ConfirmPaymentRequestCommand(requestId),
+                            cancellationToken
+                        )
+                        : await sender.Send(
+                            new RejectPaymentRequestCommand(requestId, Reason: null),
+                            cancellationToken
+                        );
+
+                await botClient.AnswerCallbackQuery(
+                    callback.Id,
+                    result.IsSuccess ? "Готово" : result.Error.Message,
+                    cancellationToken: cancellationToken
+                );
+                if (callback.Message is not null)
+                {
+                    var statusText = result.IsSuccess
+                        ? (
+                            parts[1] == "approve"
+                                ? "✅ Оплата подтверждена."
+                                : "❌ Оплата отклонена."
+                        )
+                        : $"⚠️ {result.Error.Message}";
+                    var text = $"{Escape(callback.Message.Text ?? "")}\n\n{statusText}";
+                    await botClient.EditMessageText(
+                        chatId.Value,
+                        callback.Message.Id,
+                        text,
+                        parseMode: ParseMode.Html,
+                        cancellationToken: cancellationToken
+                    );
+                }
+
+                break;
+            }
             case "cfg":
             {
                 if (!await TrySetCurrentUserAsync(services, fromId, cancellationToken))